The present invention relates to a photoelectric converter.
Recently, studies on photoelectric converters, especially solid state image pickup devices have been widely conducted as semiconductor technologies advance, and some of them have already become available.
These image pickup devices are mainly classified into those of CCD (Charge Coupled Device) type and those of MOS (Metal-Oxide-Semiconductor Device) type. For instance, the CCD type image pickup devices generally adopt the operation principle featured by forming potential wells below MOS capacitor electrodes, storing charge produced in response to incident light, and, in a readout period, sequentially shifting these potential wells in accordance with pulses to transfer, the stored charges on an output amplifier to read them out. In another CCD type image pickup device, light-receiving areas are formed by junction diodes, and transfer areas are formed by CCD structures. On the other hand, the MOS type image pickup devices adopt the operational principle featured by storing charges produced in response to incident light in respective photodiodes comprising pn junctions constituting light-receiving areas, and, in a readout period, reading out stored charges to an output amplifier by sequentially turning on MOS switching transistors respectively connected to photodiodes.
The CCD type image pickup devices are of a relatively simplified structure and, when viewed from the point of noise, only capacitance of a charge detector provided at the final stage, originating from floating diffusion, affects random noise. Accordingly, the CCD image pickup devices have relatively low noise characteristics and are capable of operating at low illumination. However, because of restriction of processes for producing CCD type image pickup devices, a MOS type amplifier serving as an output amplifier is provided on a chip and it is likely that visually noticeable 1/f noise occurs from interfaces between silicon and a film of SiO.sub.2. Accordingly, although such noise is relatively low, there exists a limitation on their performance. Further, if an attempt is made to increase the number of cells, and pack them in a high density for the purpose of performing high resolution pickup, the maximum charge storage capacity of a potential well decreases, failing to maintain a desired dynamic range. Accordingly, this will result in a big problem when solid state image pickup devices of high resolution are realized in the future Moreover, as the CCD type image pickup devices are formed to transfer stored charges by sequentially shifting potential wells, even if there exists a defect in only one cell, the charge transfer is stopped thereat or the efficiency thereof is greatly lowered, whereby it is difficult to increase production yield.
On the contrary, the MOS type image pickup devices, while they are somewhat complicated in structure as compared with the CCD type image pickup devices, particularly, the frame transfer type devices, can be so constituted that they will have a large storage capacity and a wide dynamic range. Further, even if there is a defect in one cell, the influence of the defect does not spread over the other cells because an X-Y addressing scheme is employed, whereby a high production yield is attained. However, in the MOS type image pickup devices, at the signal readout stage, wiring capacitance is connected to each photodiode, and there occurs an extremely large signal voltage drop, resulting in a low output voltage. Furthermore, noticeable random noise occurs due to a large wiring capacitance and fixed pattern noise occurs due to variation of parasitic capacitance existing in respective photodiodes and MOS switching transistors for horizontal scanning. Because of these and other difficulties, the MOS type image pickup devices have drawbacks such that it is difficult to take a picture at a low illumination, in contrast to the CCD type image pickup devices.
Further, when an attempt is made to realize high resolution image pickup devices in the future, it is expected that the dimension of each cell will be reduced and a charge stored therein will decrease. On the contrary, the wiring capacity determined by a chip size can not be lowered substantially, even if the line width becomes small. Accordingly, the MOS type image pickup devices will be further disadvantageous in respect of S/N ratio.
Although the CCD type and MOS type image pickup devices have both advantages and drawbacks described above, they are gradually approaching a practically usable level. However, they have essential problems in realizing the still higher resolution required in the future.
Meanwhile, there have been proposed novel solid-state image pickup devices as disclosed in Japanese Laid Open Patent Applications Nos. 56-150878, 56-157073 and 56-165473, all entitled "Semiconductor Image Pickup Device". While the conventional CCD and MOS type image pickup devices are based on the principle that charge generated in response to the incident light is stored in a main electrode (for instance, the source of a MOS transistor), those novel image pickup devices are based on the principle that charge generated in response to the incident light is stored in a control electrode (for instance, the base of a bipolar transistor or the gate of an SIT (static induction transistor) or MOS transistor in such a way that the flowing current is controlled in response to the charge generated in response to the incident light. That is, in the conventional CCD or MOS type image pickup devices, the stored charge per se is read out to the exterior, whereas, in those novel image pickup devices, each cell has an amplification capability so that the amplified signal is read out by charge amplification. In other words, the stored charge is read out as a low impedance output by the impedance conversion. As a result, the newly proposed image pickup devices have various advantages such as a high output, a wide dynamic range, and low noise and moreover nondestructive readout is possible because the carrier (charge) excited in response to the light or light image is stored in a control electrode. Furthermore, they can be improved in the future in such a way that they will have a higher degree of resolution.
However, these newly proposed image pickup devices fundamentally adopt an X-Y address scheme and have a fundamental cell structure comprising a cell of the conventional MOS type image pickup device and an amplifier element such as a bipolar transistor or SIT. As a result, they are complicated in construction and even though they have a possibility of exhibiting a higher degree of resolution, there exists a limit to their resolution capability at their present stage of evolution.